Air-bakeable water-proof getter device and method of manufacturing

ABSTRACT

A getter device with an organosilane coating.

Exothermic getter devices which release an evaporable getter metal suchas barium in a vacuum vessel are well known. Such getter devices usuallycomprise a pulverulent barium-aluminum alloy of approximate compositionBaAl₄ in admixture with from about 20 to 80% and preferably about 40 to60% and more preferably about 50% by weight of an additional metalcapable of reacting exothermically with the barium-aluminum alloy. Thiscorresponds to weight ratios of 4:1 to 1:4. Such co-reactant may be Ni,Ti, Fe, Mo or alloys such as intermetallic compounds of nickel withtitanium etc. However, nickel is the most preferred additional metal dueto its availability at low cost, its ease of handling, its relativestability in ambient conditions and its ability to react exothermicallywith barium-aluminum alloys with the required degree of exothermicity tocontrol the rate of barium evaporation and hence the getter deviceflashing characteristics. The use of intermetallic compounds to promotethe exothermic reaction is relatively expensive as it involves anadditional production stage in the manufacture of the exothermic getterdevice.

During the manufacture of electric discharge tubes such as color displaytubes the getter device is subjected to a wide variety of manufacturingthe environmental conditions. Exposure of the getter device to excessiveatmospheric humidity or to water during the washing of the electrodesystem of which the getter device forms a part can cause deteriorationof the materials comprising the getter device. The water or water vapormay react with the barium-aluminum alloy to produce barium hydroxidewhich may in its turn react with other constituents of the atmospheresuch as CO₂ to produce further chemical compounds. These compoundsdecompose during evaporation of the getter metal in the finished sealedelectron tube with the release of noxious gases which may influence inan undesirable way the composition of the residual gas atmosphere withinthe tube resulting in poor quality tubes having only a short workinglife.

Frequently it is desirable to have the getter device already mountedwithin the glass envelope of a color display tube during the processwhereby the phosphor bearing face-plate or screen is hermetically sealedonto the glass cone. This process generally takes place in air, ratherthan vacuum, at a temperature of about 400° C. or more. This hightemperature air exposure can again cause deterioration presumably byoxidation of one or more of the materials comprising the getter deviceto such an extent that the exothermicity of the barium release reactionbecomes so high that this reaction is almost explosive in naturereleasing barium in the form of molten particles rather than as a vaporas well as particles of getter alloy mixture. Such uncontrolled releaseof barium and other particles is detrimental as the particles can laterfall onto the electrode structure in the electron tube causingelectrical faults. In a color display tube the particles can also blockthe small apertures in the shadow mask causing areas of the phosphors tobe prevented from being excited by the electron beam with a consequentreduction in quality of the displayed image.

The use of organic materials such as nitrocellulose, methyl acrylateresins and polyimide resins dissolved in an organic solvent have beenproposed in United Kingdom Pat. No. 1,372,823 as impregnating agents toform a mechanical protective coating on the getter devices. Polyurethanecoatings have also been suggested. However, it is well known that thinmechanical coatings are very prone to the formation of pinholes andtherefore afford less protection than is desirable. Thicker pin-holefree coatings both undesirably increase the weight of the getter deviceand make it difficult to completely eliminate the solvent. Duringevaporation of the getter metal charred remains of the decomposedorganic material layer are caused to detach from the getter devicesurface causing the same inconveniences as with the evaporation ofmolten barium particles.

Most organic materials decompose at the temperatures required to sealthe face plate of a color display tube to the cone thus providing verylittle protection during this process. In this case it is usuallynecessary to replace the nickel of the getter alloy mixture with othermaterials such as the titanium-nickel intermetallic compounds to give aless exothermic reaction and avoid explosive evaporation of barium.

Furthermore, large amounts of impregnating material signify largequantities of decomposition product gases which can be harmful tocathode activity and tube life.

It is also desirable to have a single type of getter device which can besubjected to either a water washing operation or a high temperature airtreatment or both without alteration of its evaporation characteristicsso that it is not necessary to provide stocks of different devices eachcapable of being subjected to only one of the above treatments.

It is therefore an object of the present invention to provide animproved getter device and an improved process for the manufacture of anexothermic getter device which is substantially free from one or more ofthe disadvantages of the prior art.

Another object is to provide an exothermic getter device which does notdeteriorate on exposure to water or water vapor.

Yet another object is to provide an exothermic getter device which doesnot deteriorate on exposure to high temperatures in air.

A still further object is to provide an exothermic getter device whichdoes not substantially alter the residual gas atmosphere within anelectric discharge tube after evaporation of the getter metal.

Additional objects and advantages of the present invention will beapparent to those skilled in the art by reference to the followingdetailed description and drawings wherein:

FIG. 1 is a cross-section of a getter device according to the presentinvention.

FIG. 2 is a diagrammatic cross-section of an apparatus for performingthe process of the present invention.

FIG. 3 is a diagrammatic partial cross-section of a color display tubehaving two exothermic getter devices of the present invention.

FIG. 4 is Formula I.

FIG. 5 is Formula II.

FIG. 6 is Formula III.

According to the present invention there is provided a method ofmanufacturing an air-bakeable, water-proof, exothermic getter device,and an electric discharge tube manufactured by using such a getterdevice. The method of the present invention comprises the steps ofmanufacturing a getter device by pressing a pulverulent mixturecomprising a barium alloy and preferably a barium-aluminum alloytogether with a metal capable of reacting exothermically with thebarium-aluminum alloy into a holder, then the pulverulent mixture in theholder is exposed to an organosilane. In the broadest sense of thepresent invention any method of contacting the getter device with theorganosilane is suitable. For instance in one preferred method thegetter device may be dipped into a bath of the organosilane present as apure liquid or in mixture with a suitable solvent. Another preferredmethod is to place the getter device in a vessel together with an amountof the organosilane. The vessel is evacuated and sealed. Theorganosilane is then heated to raise its vapor pressure so that itsvapor molecules may contact the surface of the getter device.

The getter device is generally held at a temperature higher than that ofthe bath of the organosilane so that no liquid condensation may takeplace on the devices.

Furthermore, the temperature of the getter devices should be such that amolecular layer of a cross-linked organosilane coating forms on thesurface of the getter device. The formation of such layers has beendescribed in U.S. Pat. No. 3,901,761, however, this patent gives noindication that such a monomolecular layer provided on an exothermicgetter device would be capable of preventing explosive evaporation ofbarium after water exposure or water vapor exposure or a hightemperature air exposure.

Preferably the temperature of the getter device should be kept higherthan about 100° C. in order that the organosilane may form across-linked monomolecular layer chemically bonded to the surfaces ofthe getter device. The highest temperature at which the getter deviceshould be maintained is that temperature at which barium commences toevaporate, which is about 700° C. to 750° C. However, it is convenientto maintain the getter device at a temperature below the flash point ofthe particular organosilane being used. This ensures that no explosivereaction takes place between organosilane vapors and atmospheric oxygenin case of accidental breakage of or leaks in the evacuated vessel.

In addition to the barium alloy and the exothermic co-reactant thegetter device can further comprise gas releasing materials such as Fe₄ Nor TiH₂ as well as thermic moderators such as tungsten.

In accordance with the broadest aspects of the present invention anyorganosilane can be employed which imparts the herein describedproperties to the getter devices. Examples of suitable classes oforganosilanes include, among others, monomeric siloxanes, polysiloxanes,aliphatic silanes and aromatic silanes and the substitution productsthereof.

A preferred class of polysiloxanes are those of Formula I shown in FIG.4 wherein R¹, R², R³, R⁶, R⁷ and R⁸ are independently selected from thegroup consisting of alkyl, aryl, aralkyl, alkaryl and hydrogen, with theproviso that at least one is selected from the group consisting ofalkyl, aryl, aralkyl and alkaryl; wherein R⁴ and R⁵ are independentlyselected from the group consisting of alkyl, aryl, aralkyl and alkaryl;wherein "n" is generally an integer from zero to twenty thousandinclusive and is preferably an integer from zero to one thousandinclusive. Preferred species of polysiloxanes of Formula I are shown inFormulas II and III in FIGS. 5 and 6 respectively.

Examples of suitable alkyl radicals include, among others, methyl,ethyl, propyl, t-butyl, dodecyl, and allyl. Examples of suitable arylradicals include, among others, phenyl and naphthyl. Examples ofsuitable aralkyl radicals include, among others, benzyl and2-phenylpropyl. Examples of suitable alkaryl radicals include amongothers 3-methyl-phenyl and 4-butylnapthyl. Any of the above can besubstituted by one or more non-interfering radicals such as halogens,such as chlorine or bromine. However, there is a widely held belief inthe electronic industry that halogens have a damaging effect on thecathode activity of electronic discharge tubes. For this reason it ispreferred to use non-halogenated organosilanes.

Referring now to the drawings and in particular to FIG. 1, there isshown an exothermic getter device 10 of the present invention.Exothermic getter device 10 comprises a stainless steel holder 11 in theform of an annular ring having a U-shaped channel cross section. Withinthe holder 11 is a pulverulent barium-aluminum alloy 12 mixed withpulverulent nickel 13. A cross-linked organosilane coating 14 isthermally bonded to the surface of the powders 12, 13.

FIG. 2 shows schematically, an apparatus 20 suitable for performing thepreferred process of the present invention. Apparatus 20 comprises aglass vessel 21 evacuated and sealed at one end 22 by normal glassblowing techniques. Within vessel 21 is a wired support structure 23supporting a number of exothermic getter devices 10', 10", 10'".Furthermore the glass vessel 21 contains a non-halogenated organosilanefluid 24.

Surrounding the portion of vessel 21, which contains the organosilanefluid 24, is placed in a first heating device 26. A second heatingdevice 26 in the form of a spiral coil of resistance wire is placedaround vessel 21 to heat the exothermic getter devices 10', 10", 10'". Acooling coil 27 is placed around vessel 21 in a zone between the firstand second heating devices to ensure that condensation as a liquid ofthe organosilane vapor takes place preferentially in this zone ratherthan on the exothermic getter devices 10', 10", 10'".

In operation the getter devices 10', 10", 10'" are heated to about 200°C. by means of coil 26. The organosilane fluid is heated to about 200°C. by means of the heating device 25. Cooling coil 27 maintains the zonebetween the two heating devices at about 180° C. The getter devices aremaintained at 200° C. for 1 hour after which the apparatus is allowed tocool to room temperature and the getter devices are removed from thevessel. They are now ready to be used in the fabrication of electricdischarge tubes such as a color display tube.

FIG. 3 shows a partial cross-section of a color display tube 30comprising a shadow mask 31, an electron gun assembly 32 and an ironscreening element 33. A first exothermic getter device 10' treatedaccording to the above organosilane process is assembled onto theelectron gun. The getter device and electron gun are previously washedin tepid water, which may also contain a small amount of surface activeagent. A second getter device 10", again treated according to the aboveprocess, is attached to the iron screen 33. The second getter device 10"does not have to be washed but it may remain exposed to the atmosphereand hence humidity for relatively long times before final tube assembly.Furthermore, the second getter device 10" is already attached to ironscreen 33 during sealing of the face plate 34 to the cone 35 by means ofthe hermetic seal 36. This sealing process takes place at 400° C. ormore and lasts for 1 hour.

The invention is further illustrated by the following examples in whichall parts and percentages are by weight unless otherwise indicated.These non-limiting examples are illustrative of certain embodimentsdesigned to teach those skilled in the art how to practice the inventionand to represent the best mode contemplated for carrying out theinvention.

EXAMPLE 1

Six identical exothermic getter devices were manufactured eachcomprising a stainless steel holder in the form of a ring of 18 mmoutside diameter having a U-shaped channel cross-section. In eachchannel were placed 320 mg of a pulverulent mixture, of approximately50% by weight of an alloy of 50% by weight barium and aluminum, with 50%by weight of nickel. Three of the getter devices were placed on a wiresupport within a glass vessel. Approximately 10 cc of trimethylpenta-phenyl tri-siloxane was also placed in the glass vessel, out ofdirect contact with the getter devices. The glass vessel was evacuatedby a rotary pump and then sealed. The sealed vessel was then placed inan oven at 200° C. for 1 hours. After removal from the oven the vesseland its contents were allowed to cool to room temperature and thetreated getter devices were removed.

All six getter devices were then heated in air at 420° C. for 1 hour tosimulate the color display tube face plate to cone sealing process.

The six getter devices were each placed in separate glass vessels, whichwere then evacuated by a rotary and a diffusion pump. Each getter devicewas then caused to evaporate barium by subjecting them to inductionheating. The three getter devices which had not been treated with theorganosilane fluid exhibited an explosive evaporation of barium withparticles of barium and the getter alloy mixture being violently ejectedfrom the stainless steel holder. The three getter devices treated withorganosilane fluid showed a controlled evaporation of barium vapor withno signs of explosive reaction.

EXAMPLE 2

A further six getter devices were prepared as in Example 1, three ofwhich were treated with tri-methyl pentaphenyl tri-siloxane exactly asdescribed in Example 1. The getter devices were then immersed in warmtap water. After about 20 seconds, the untreated getter devices startedto evolve bubbles of gas, thus showing attack of the getter metal vaporreleasing material by the water. Even after 1 minute the treated getterdevices showed no signs of attack as no gas bubbles were evolved.

EXAMPLE 3

Getter devices were prepared as described in Example 1. The getterdevices were immersed in di-methyl polysiloxane oil (AK 0.65manufactured by Wacker-Chemie GmbH, Monaco, Germany) by immersion in theoil for 15 minutes. The impregnated getter devices were then air-dried.

The getter devices were then heated in air at 420° C. for 1 hour and"flashed" in a vacuum as described in Example 1.

The getter devices treated with di-methyl polysiloxane fluid showed acontrolled evaporation of barium vapor with no signs of explosivereaction.

EXAMPLE 4

Getter devices were prepared as in Example 3. After being air-dried, thegetter devices were treated thermally in dynamic vacuum at 300° C. for60 minutes. Then the getter devices were heated in air at 420° for 1hour and "flashed" in a vacuum as described in Example 3. The resultswere similar to those of Example 3.

EXAMPLE 5

Getter devices were prepared as in Example 3. After being air-dried thegetter devices were placed in warm water (about 60° C.) and the timefrom immersion to copious bubble formation was observed visually.Bubbles formed rapidly on the getter devices.

EXAMPLE 6

Getter devices were prepared as in Example 4. After the thermaltreatment in dynamic vacuum at 300° C. for 60 minutes the getter deviceswere washed with warm water as in Example 5. Bubbles formed on thegetter devices after 21/2 minutes.

EXAMPLES 7-10

Examples 3-6 were repeated using 10% by volume of thedimethyl-polysiloxane oil with 90% by volume of xylene to impregnate thegetters. Results were similar to those using 100% oil.

EXAMPLES 11-14

Examples 7-10 were repeated using a dimethyl polysiloxane oil with achain length having approximately 200 silicon-oxygen groups permolecule. This oil of longer chain length proved preferable to the oneused for Examples 7-10, as the getter devices showed greater resistancein the "H₂ O test".

EXAMPLES 15-18

Examples 7-10 were repeated using a dimethylpolysiloxane oil with achain length having approximately 1400 silicon-oxygen groups permolecule. The results were similar to those found in Examples 11-14except for a failure (rapid bubble formation) in Example 17.

EXAMPLES 19-22

Examples 11-14 were repeated using a methyl phenyl polysiloxane oil witha chain length of approximately 108 silicon-oxygen groups per moleculeand approximately a 12% phenyl substitution in the chain. The resultswere similar to those obtained in Examples 11-14, though with betterresults in Example 21 compared with the similar Example 13.

EXAMPLES 23-24

Examples 1 and 2 were repeated but using the same methyl phenylpolysiloxane used in Examples 19-22. The results were similar to thoseof Examples 1 and 2.

EXAMPLES 25-28

These examples were carried out using methyl dichlorosilane (No. 5899manufactured by Merck and Co., Inc., New Jersey, U.S.A.). The getterdevices were exposed to the methyl dichlorosilane vapor in air for 60seconds and then tested similarly to Examples 7-10. The results of thetests were all satisfactory.

EXAMPLES 29-32

Examples 25-28 were repeated using dichlorochlormethyl methyl silane(No. 804/86 Merck and Co., Inc., N.J., U.S.A.). The getter devicesformed bubbles rapidly during the immersion in water test, but performedsuccessfully in the "Frit" test.

EXAMPLE 33

This comparative example records the behavior of additional prior-art,untreated control getters as in Example 1.

EXAMPLE 34

This records the behavior of untreated control getters used in each ofthe water washing tests described in the foregoing examples.

The above examples are summarized in Table 1 below, with its appendedexplanatory footnotes.

    __________________________________________________________________________    TESTS PERFORMED                                                                                %      H.sub.2 O                                                                         FRIT         EXAMPLE                              "n"                                                                              OIL   BULB                                                                              BATH                                                                              OIL                                                                              BAKE                                                                              TEST                                                                              TEST                                                                              COMMENTS NO.                                  __________________________________________________________________________    1  DC 705                                                                              X                  X   OK-no explosion                                                                        1                                    1  DC 705                                                                              X              X       OK-resists more                                                                        2                                                                    than 5 min.                                   0  AK 0.65   X   100        X   OK-no explosion                                                                        3                                    0  AK 0.65   X   100                                                                              X       X   OK-no explosion                                                                        4                                    0  AK 0.65   X   100    X       fail     5                                    0  AK 0.65   X   100                                                                              X   X       resists 21/2                                                                           6                                                                    min.                                          0  AK 0.65   X   10         X   OK-no explosion                                                                        7                                    0  AK 0.65   X   10 X       X   OK-no explosion                                                                        8                                    0  AK 0.65   X   10     X       fail     9                                    0  AK 0.65   X   10 X   X       resists 21/2                                                                           10                                                                   min.                                          200                                                                              AK 350    X   10         X   OK-no explosion                                                                        11                                   200                                                                              AK 350    X   10 X       X   OK-no explosion                                                                        12                                   200                                                                              AK 350    X   10     X       resists 3 min.                                                                         13                                   200                                                                              AK 350    X   10 X   X       OK-resists more                                                                        14                                                                   than 5 min.                                   1400                                                                             AK 100,000                                                                              X   10         X   OK-no explosion                                                                        15                                   1400                                                                             AK 100,000                                                                              X   10 X       X   OK-no explosion                                                                        16                                   1400                                                                             AK 100,000                                                                              X   10     X       fail     17                                   1400                                                                             AK 100,000                                                                              X   10 X   X       OK-resists more                                                                        18                                                                   than 5 min.                                   108?                                                                             AS 200    X   10         X   OK-no explosion                                                                        19                                   108?                                                                             AS 200    X   10 X       X   OK-no explosion                                                                        20                                   108?                                                                             AS 200    X   10     X       OK-resists more                                                                        21                                                                   than 5 min.                                   108?                                                                             AS 200    X   10 X   X       OK-resists more                                                                        22                                                                   than 5 min.                                   108?                                                                             AS 200                                                                              X                  X   OK-no explosion                                                                        23                                   108?                                                                             AS 200                                                                              X              X       OK-resists more                                                                        24                                                                   than 5 min.                                   5899     Vapour in Air      X   OK-no explosion                                                                        25                                   5899     "          X       X   OK-no explosion                                                                        26                                   5899     "              X       OK-resists more                                                                        27                                                                   than 5 min.                                   5899     "          X   X       OK-resists more                                                                        28                                                                   than 5 min.                                   804/86   "              X       OK-no explosion                                                                        29                                   804/86   "          X       X   OK-no explosion                                                                        30                                   804/86   "              X       fail     31                                   804/86   "          X   X       fail     32                                                               X   fail-explodes                                                                          33                                                           X       fail-resists only                                                                      34                                                                   20 sec.                                       __________________________________________________________________________

In the table "n" is the approximate number of silicon-oxygen groups permolecule of polysiloxane as shown in Formula I.

In the table "oil" refers to polysiloxane and chlorsilane manufacturers'identification number. The number following the symbols AK and ASidentifies the viscosity of the oil in centistokes

In the table "bulb" identifies the method of impregnating the getterdevices using the pure oil in an evacuated glass vessel.

In the table "bath" identifies the method of impregnating the getterdevices by immersing them in the oil or oil-solvent mixture for 15minutes.

In the table "% oil" gives volume % oil to xylene composition of thebath.

In the table "bake" refers to the thermal treatment in dynamic vacuum at300° C. for 60 minutes after air-dry.

In the table in the "H₂ O test" getter devices are placed in warm water(approx. 60° C.) and the time until bubble formation is observedvisually.

The "Frit test" referred to in the table is an air heat treatment for 1hour at 420° C. and subsequent "flash" in vacuum. "Explosion" isobserved by visual observation of vigorous ejection of glowing particlesfrom the getter device.

From the results obtained in the examples as described above, it can beseen, by comparing Examples 3-6 with Examples 7-10, that use of a pureoil or an oil solvent mixture gives identical results.

Comparison of Examples 5, 9, 13, 17 show that without the thermal vacuumtreatment ("bake") only oils of "n" about 108 give any substantialdegree of water treatment protection. Comparison of Examples 6, 10, 14,18 show that the "bake" treatment improved the water resistanceconsiderably being satisfactory for "n" greater than zero. It can beseen from the results of Examples 19-22 that for methyl-phenyl oils the"bake" treatment may not be necessary.

Examples 3, 4, 7, 8, 11, 12, 15, 16 show that the oil-treated getterdevices are "frittable" both with and without the "bake" treatment.

Comparison of Examples 19-22 with Examples 23-24 show no differencebetween the efficiency of "bulb" and "bath" treatments.

It can be seen from the results of Examples 25-28 compared with Examples29-32 that the halogenated silanes are rather selective in giving waterprotection.

The oils used in Examples 19-22 had approximately 10% phenylsubstitution while Examples 1-2 had 62.5% phenyl substitution.Comparison shows similar results with both amounts of phenyl in thechain and for both lengths of the chain of siloxane groups (108 forExamples 19-22, 3 for Examples 1-2).

Although the invention has been described in considerable detail withreference to certain preferred embodiments thereof, it will beunderstood that variations and modifications can be effected within thespirit and scope of the invention as described above and as defined inthe appended claims.

What is claimed is:
 1. A getter device comprising a barium alloy; ametal capable of reacting exothermically admixed with the barium alloyand having an organosilane coating.
 2. A getter device of claim 1wherein the organosilane is a polysiloxane.
 3. A getter device of claim2 wherein the polysiloxane is one of Formula I ##STR1## wherein R¹, R²,R³, R⁶, R⁷, and R⁸ are independently selected from the group consistingof alkyl, aryl, aralkyl, alkaryl and hydrogen with the proviso that atleast one is selected from the group consisting of alkyl, aryl, aralkyl,and alkaryl;wherein R⁴, and R⁵ are independently selected from the groupconsisting of alkyl, aryl, aralkyl and alkaryl; wherein "n" is aninteger from zero to 20,000 inclusive.
 4. A getter device of claim 1wherein the organosilane is an aliphatic or aromatic silane.
 5. A getterdevice of claim 1 wherein the barium alloy is a barium-aluminum alloy.6. A getter device of claim 5 wherein the barium-aluminum alloy isBaAl₄.
 7. A getter device of claim 1 wherein the metal capable ofreacting exothermically is selected from the group consisting of Ni, Ti,Fe, Mo and alloys thereof.
 8. A getter device of claim 1 wherein themetal capable of reacting exothermically is Ni.
 9. A getter device ofclaim 1 comprising:A. an annular ring; B. a pulverulent intimate mixtureof a barium-aluminium alloy and nickel held by the ring; C. anorganosilane coating covering the entire getter device wherein saidcoating is an organosilane of Formula II ##STR2##
 10. A getter device ofclaim 1 comprising:A. an annular ring; B. a pulverulent intimate mixtureof a barium-aluminium alloy and nickel held by the ring; C. anorganosilane coating covering the entire getter device wherein saidcoating is an organosilane of Formula III ##STR3##
 11. A getter devicecomprising a barium alloy, a metal capable of reacting exothermicallyadmixed with the barium alloy and having an organosilane coating,wherein the weight ratio of the barium alloy to the metal capable ofreacting exothermically is 1:4 to 4:1.
 12. A method of manufacturing anexothermic getter device comprising the steps of pressing a pulverulentmixture comprising a barium-aluminum alloy together with a metal capableof reacting exothermically with the barium-aluminum alloy into a holder,then exposing the pulverulent mixture in the holder to an organosilane,thereby producing a coating.
 13. A method according to claim 12 in whichthe organosilane contains aliphatic and/or aromatic radicals.
 14. Amethod according to claim 12 in which the organosilane is poly di-methylsiloxane.
 15. A method according to claim 12 in which th e organosilanecontains phenyl radicals.
 16. A method according to claim 15 in whichthe organosilane is selected from the group consisting of:(a)Tetra-methyl tetra-phenyl tri-siloxane (b) Tri-methyl penta-phenyltri-siloxane.
 17. A method according to claim 12 in which theorganosilane contains both phenyl and methyl radicals.
 18. A methodaccording to claim 12 in which the organosilane is in the vapor phase.19. A method according to claim 12 in which the organosilane is in theliquid phase.